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    Re: Corrections for air bubble sextants
    From: David Pike
    Date: 2022 Jul 31, 09:30 -0700

    Juanjo Garrido you wrote: I got a Kollsman periscopic sextant and a Mk. IX….. Could you help me …….

    Juango
    A few home truths about bubble sextants.
    1.  Don’t be put off by the amazing accuracies attributed to marine sextant sights on this forum.  Aircraft sextants were never meant to be that accurate.  Just look inside your MkIX (at least 70 years old since first cast or pressed) or imagine the inside of your Kollsman and imagine all the linkages, slots, stops, cams, and gearwheels. Then look at that aluminium tube gleaming in the Sun or losing heat to the winter frost.  If you’ve got power connected, is any going through the internal heaters in the sextant (not the Mk IX) and the mounting? If so, how long has it been on for? Is it starting to feel warm?  With a bubble sextant stationary on land, anything less than 3nm is good.  In the air, 10nm is not uncommon.  OK, if you know exactly where you are, you can work out your index error, but don’t expect it to be the same tomorrow.

    2.  Averagers are there to even out acceleration errors, which are not normally present when stationary, so we could argue for weeks whether it’s worth using the averager with the ground under our feet. Personally, I've not got the patience.  Timed mean averagers like the Hughes RAF MkIXA,AM,B, and BM' i.e. one or two recordings per second' are preferable to median averagers like many WW2 US bubble sextants or taking the mean of several single shots like the original MkIX (with no letters after the X), where the observer was tempted to only press the trigger when the bubble appeared steady.  This is because it’s the rate of change of acceleration which affects bubble movement.  Bubble movement appears steadiest when the acceleration is at its maximum or minimum and greatest as the acceleration slices through zero.  Therefore, with a regular recording mean averager, you must always try and follow the bubble.

    3. Another possible problem with averagers occurs when the body’s azimuth is 180 or 360 when the body will be at its maximum or minimum altitude although the effect is likely to be small, because the body’s altitude is hardly changing at those azimuths.

    4. They’re great fun though.  I love them. DaveP

       
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